The heat flow equation shows that geothermal gradient varies directly with conductive heat flow and inversely with thermal conductivity. Because of the second of these functions, geothermal gradient values have limited validity wherever bottom-hole temperature (BHT) readings are reduced without regard for ambient rock type. Thus, I present a geothermal gradient map of Texas using new data from selected geologic formations of generally uniform rock type. Four carbonate rock units provide the data base for most of the state inland of the Lower Cretaceous shelf edge. These include, for the inner Gulf Coast Basin region, the Jurassic Smackover Formation and the Cretaceous Sligo and Edwards Formations. For most of the remainder of the state west of the Balcones/Ouachita trend, data were obtained for the Ordovician Ellenburger Group. Coastward of the Cretaceous shelf edge, data from the 1976 AAPG and U. S. Geol. Survey map were employed owing to the paucity

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of carbonate-rock horizons at reasonable depths there.

Despite this selective use of BHT data, the geothermal gradient contours do not necessarily represent areal variations in heat flow. Since all the horizons charted are petroleum reservoirs (and in their updip reaches, some are also aquifers), hydrodynamics also must be involved. High gradient anomalies may thus be due to upwelling of basinal waters; low anomalies may be a result of recharge. Hydrodynamic influences also provide a local means for delimiting structures, with uplifts commonly being the loci of upwelling and thus registering relatively high geothermal gradients. High anomalies are even more consistently correlated with fault zones, as seen along the Luling-Mexia trend in south-central Texas where there is an overlap of gradient highs among the three horizons contoured.